Το κεντράρισμα είναι σωστό, μόνο που το πανωτρόχι είναι βρεγμένο.

Because both nouns are singular neuter and definite.

• το κεντράρισμα = the centering / the alignment
• το πανωτρόχι = the upper wheel / the upper roller (depending on context)

Greek uses the definite article much more often than English, including with technical or abstract nouns. So even where English might drop the, Greek often keeps it.

They end in -ο because they are agreeing with neuter singular nouns.

In Greek, adjectives must match the noun in gender, number, and case.

Here:

• κεντράρισμα is neuter singular, so: σωστό
• πανωτρόχι is neuter singular, so: βρεγμένο

Compare the forms:

• masculine: σωστός, βρεγμένος
• feminine: σωστή, βρεγμένη
• neuter: σωστό, βρεγμένο

Here μόνο που means something like except that, only that, or it’s just that.

It introduces a qualification to what was just said:

• Το κεντράρισμα είναι σωστό = the centering is correct
• μόνο που... = except that / the only problem is that...
• το πανωτρόχι είναι βρεγμένο = the upper wheel/roller is wet

So the sense is: Everything is fine in that respect, except for this one issue.

Both can connect contrasting ideas, but they do not feel exactly the same.

• αλλά = but
• μόνο που = except that / it’s just that

αλλά is a more direct contrast.
μόνο που often sounds softer, as if you are saying: yes, that part is true, the only issue is...

So in this sentence, μόνο που fits well because the speaker is not rejecting the first statement. The speaker is saying the first statement is true, with one complication.

Because the sentence contains two complete clauses, and each clause has its own predicate.

1. Το κεντράρισμα είναι σωστό
2. το πανωτρόχι είναι βρεγμένο

Greek normally keeps είναι in both clauses here. English sometimes allows more ellipsis, but in this sentence Greek strongly prefers repeating the verb.

It is a noun.

It comes from the verb κεντράρω and uses the noun-forming ending -μα / -ισμα, which often creates nouns referring to an action, process, or result.

So κεντράρισμα means something like:

• the centering
• the alignment
• the act/result of centering

This is very common in Greek. Similar patterns include forms like:

• παρκάρω → παρκάρισμα
• καθαρίζω → καθάρισμα

It is the neuter singular form of βρεγμένος, which historically is a participle-like adjective meaning wet.

In practice, for a learner, it is best to treat it here as a normal adjective:

• είναι βρεγμένο = it is wet

So grammatically it behaves like an adjective, even though its origin is participial.

Because these are two different structures.

• το κεντράρισμα είναι σωστό = the centering is correct
  This is a full sentence with subject + verb + predicate adjective.

• το σωστό κεντράρισμα = the correct centering
  This is only a noun phrase, not a full statement.

So the sentence needs είναι σωστό because the speaker is making a statement, not just naming a thing.

They are both in the nominative singular.

That is because each one is the subject of είναι:

• Το κεντράρισμα είναι σωστό
• το πανωτρόχι είναι βρεγμένο

The predicate adjectives σωστό and βρεγμένο also appear in the matching neuter singular form.

The given word order is the most natural neutral order.

Greek word order is more flexible than English, but changing it usually changes the emphasis. The sentence as written presents:

1. the main statement first
2. the qualification after it

That is exactly what μόνο που is good at doing. A different order is possible in some contexts, but it would sound more marked or more emphatic. For a learner, the given order is the safest one to use.

The written accent mark shows the stressed syllable:

• κεντράρισμα → stress on τρά
• σωστό → stress on στό
• πανωτρόχι → stress on τρό
• βρεγμένο → stress on γμέ

Stress matters in Greek, so it is worth learning each word together with its accent.